Aside from addiction, what I've noticed as a parent and teacher about kids who have a lot of screen time (particularly video games, but to some extent social media) is that they have a reduced ability to deal with the real world. Kids who are heroes performing superhuman athletic feats onscreen discover that real hills are actually steep, water is actually wet, etc. These tend to be the kids who have the hardest time, for example, on a field trip doing a moderate amount of walking. This carries over into their schoolwork, where they often have a hard time persisting without the constant rewards and entertainment built into video games. My general observation, both from the classroom and from my own kids' friends and neighbors, is that more than an hour or so a day of video game time seems to reduce kids' maturity and competence in general.